This invention relates to an auxiliary air quantity control system for internal combustion engines at deceleration, and more particularly to a system of this type which is intended to properly controlling an amount of auxiliary air supplied to the engine at deceleration thereof in dependence upon the reduction ratio of a transmission connected to the engine.
A control system for controlling the auxiliary air amount is known e.g. from Japanese Provisional Patent Publication (Kokai) No. 63-18152, in which while the throttle valve is open, the opening of a control valve for opening and closing a bypass passage bypassing a throttle valve of the engine is set to a value corresponding to the opening of the throttle valve, whereas When the throttle valve is closed, the opening of the control valve is gradually decreased from a value assumed immediately before the throttle valve becomes closed, so as to gradually decrease an amount of intake air supplied to the engine, to thereby prevent sudden atomization of fuel adhering to the inner surface of the intake pipe of the engine due to suddenly increased vacuum within the intake pipe by the closure of the throttle valve, and hence prevent overriching of the air-fuel ratio of an air-fuel mixture supplied to the engine.
To be specific, the above conventional control system is constructed such that when the opening of the throttle valve is smaller than a predetermined value, the control valve is kept closed even if the throttle valve is open, whereby the intake air is supplied to the engine only through the throttle valve. This construction is based on the following ground:
When the throttle valve is suddenly closed from a state in which it is slightly open, vacuum within the intake pipe increases at so small a rate that the degree of overriching of the air-fuel ratio of the mixture is very small to make it unnecessary to gradually decrease the intake air or auxiliary air amount after the throttle valve is fully closed. Further, when the throttle valve is closed to decelerate the vehicle while the transmission is in a small reduction ratio position, e.g. in the fourth speed gear position, in order to obtain good decelerability of the engine the control valve should be closed to suddenly decrease the auxiliary air amount to thereby properly decrease engine output before and after the throttle valve is fully closed.
However, the above construction has the disadvantage that when the transmission is in a large reduction ratio position, e.g. in the first speed position, if the auxiliary air amount is suddenly decreased before and after the throttle valve is fully closed, the engine torque suddenly decreases by a large amount to cause engine shock at the start of deceleration.